This invention relates to a continuously variable transmission system and more particularly to a drive belt ratio measurement system.
A continuously variable transmission ("CVT"), generally utilizes a pair of adjustable pulleys, including a primary pulley and a secondary pulley. The primary pulley is mounted on an input shaft connected to an engine. In certain embodiments the primary pulley is connected to the engine through a torque converter. The secondary pulley is mounted on an output shaft connected to the drive train of the vehicle, typically through a clutch. One example of such a CVT is shown in Smirl, U.S. Pat. No. 4,433,594 entitled "Variable Pulley Transmission" which is incorporated herein by reference. A drive belt interconnects the pulleys and transfers power from the engine to the drive train through the frictional contact between the side faces of the drive belt and the contact faces of the adjustable pulleys. The side faces of the drive belt are shaped to correspond to the contact faces of the pulleys.
Each pulley includes two sheaves or flanges having conical side surfaces defining a V-shaped gap between the sheaves. At least one of the sheaves is moveable along the axis of the shaft so that the gap between the sheaves can be varied. When the gap width between the sheaves varies, the radial contact point of the drive belt also varies due to the conical shape of the side surfaces of the sheaves. Consequently, the effective radius of the pulleys vary along with the transmission drive ratio. Generally, as the effective diameter of the primary pulley is moved in one direction, the effective diameter of the secondary pulley is moved in the opposite direction. This permits a continuous adjustment of the drive ratio between the engine and the drive train, while maintaining the tautness of the belt.
Typically, the transmission belt ratio is determined by measuring the speed of the primary and secondary sheaves. The transmission belt ratio is then calculated by mathematically computing the ratio of the primary pulley's speed to the secondary pulley's speed. If, however, clamping forces of the sheaves on the chain belt are insufficient for the torque being transmitted, the belt may slip on the sheaves. This is undesirable because the efficiency of the transmission is greatly compromised. Although the sheave speed method of calculating the belt ratio adequately provides the actual belt ratio, it alone cannot be used to determine if belt slippage is occurring or if the system is operating at peak efficiency.
The present invention solves this problem by providing a mechanism that can determine the theoretical belt ratio. The mechanism can also compare the actual belt ratio to the theoretical belt ratio to anticipate an impending belt slip condition so that the appropriate corrective action can be taken to stop belt slippage and to prevent further belt slippage.